Lower olefins (C2-C4 olefins) are the fundamental starting materials for the organic chemical industry, and have an important role in the modern petroleum and chemical industry. On the whole, the process for producing light olefins may be divided into two general classes, i.e. the traditional petroleum way and the novel non-petroleum way. Since the 1910s, the world began to investigate the process for producing light olefins from non-petroleum resources (especially the oxygen-containing compound feedstock) and made some progresses.
The process of producing light olefins through the dehydration reaction of the oxygen-containing compound produces a certain amount of water as by-product except the hydrocarbon products, for example, about 44% of the hydrocarbon products from methanol and about 46% of the hydrocarbon products from ethanol. It is known that the reaction producing light olefins with the oxygen-containing compound feedstock is a reaction in which the amount of molecules increases, and therefore the lower reaction pressure is favorable for the chemical equilibrium to proceed toward the production of light olefins. Therefore, upon producing light olefins, in order to obtain a desired yield of light olefins, the prior art generally uses a lower reaction pressure. This lower reaction pressure (typically 0.1-0.3 MPa) directly results in that if desired to increase the throughput of the oxygen-containing compound feedstock in order to increase the output of light olefins, the prior art will therefore have to increase the size or amount of the reactor so as to maintain the yield of light olefins at an acceptable level. Obviously, this will accordingly increase the investment and maintenance cost of the plant.
In the process for producing light olefins according to the prior art, in order to guarantee a continuous production process, the catalyst is circulated between the reactor and the regenerator. In order to facilitate the circulation, the reactor and the regenerator are generally operated at the substantially same pressure. Under this situation, the reactor is in a hydrocarbon atmosphere, and the regenerator is in an oxygen-containing atmosphere. If the reactor and the regenerator are not well segregated, there will be a large potential safety hazard.
In addition, a cyclone similar to that used in the catalytic cracking unit is widely used in the plant for producing light olefins according to the prior art. Therefore, it is inevitable for the catalyst natural loss during the production, in particular in case that the catalyst fine powder having a particle size of less than 20 microns becomes more and more in the catalyst. This will have a detrimental effect on the subsequent product separation, and will be adverse for the catalyst to be reused.